By way of example, the substrate may be a superalloy, in particular a superalloy for constituting turbomachine parts.
The two technologies that are used industrially for depositing, onto a substrate, a material that acts as a thermal barrier, typically a ceramic, are plasma spraying, and vapor phase deposition.
Plasma spraying consists in injecting the material for deposition in powder form into the plasma jet of a plasma torch. The plasma jet is generated by creating an electric arc between the anode and the cathode of a plasma torch, thereby ionizing the gaseous mixture blown through said arc by the plasma torch. The size of the powder particles injected into the jet lies typically in the range 1 micrometer (μm) to 50 μm. The plasma jet, which reaches a temperature of 20,000 K and a speed of the order of 400 meters per second (m/s) to 1000 m/s entrains and melts the powder particles. They then strike the substrate in the form of droplets which, on impact, solidify in a flattened shape.
Vapor phase deposition generally makes use of an electron beam for vaporizing the material that is to be deposited. The most widespread technique is electron beam physical vapor deposition (EBPVD). Once the material has been vaporized by the electron beam, it condenses on the substrate. Because a beam of electrons is used, it is necessary to maintain a secondary vacuum inside the enclosure that contains the electron beam, the material to be deposited, and the substrate.
Other technologies exist, but they are not yet at an industrial stage. Electron beam directed vapor deposition (EBDVD) is based on the same principle as EBPVD. Thermal plasma physical vapor deposition (TPPVD) uses a plasma torch as a source of heat to evaporate the material that is to be deposited. The torch is coupled to a radiofrequency source for increased efficiency. The technical obstacle posed by that method is keeping the powder of the material for deposition in the plasma for a length of time that is long enough for it to vaporize.
Each of the two technologies used industrially for depositing, onto a substrate, a material that acts as a thermal barrier possesses advantages and drawbacks:
The deposit that results from plasma spraying presents lamellar morphology, the superposed lamellae being parallel to the surface of the substrate. The deposit possesses microcracks that are due to the quenching of the droplets while they are being subjected to impact on the substrate, so the deposit is porous. Because of its structure and its porosity, the deposit thus has the advantage of possessing low thermal conductivity. The substrate is thus better protected thermally. However, that type of deposit presents limited lifetime since thermal expansions of the substrate tend to fracture the deposit and cause it to spall. It is also difficult with that method to obtain a deposit of uniform thickness on parts that are complex in shape, since the method is highly directional.
The deposit that results from electron beam vapor phase techniques presents columnar morphology, the columns being arranged beside one another perpendicularly to the surface of the substrate. The deposit thus presents good lifetime, firstly because its structure accommodates thermal expansion of the substrate well, and secondly because its resistance to erosion is much greater than that of a plasma deposit. However, the deposit possesses thermal conductivity that is higher than that of a deposit obtained by plasma spraying, which is undesirable since the deposit then constitutes a thermal barrier that is less effective. In addition, deposition rate and yield are low. The low yield is due to the fact that the method creates a “cloud” of vapor, which therefore condenses in indiscriminant manner, including on the walls. Above all, electron beam deposition is a technique that is expensive and difficult, since it requires high levels of electrical power for the electron guns and to obtain a high vacuum in enclosures of large volume.